The film opens in Japan in another dimension, where Godzilla rises out of the sea to wreak havoc on Tokyo once again, destroying everything in his path. The Japanese military throws everything they have at the beast, but it is ineffective against the monster and Godzilla just blasts the entire military force with his atomic breath.
Meanwhile, in the Marvel Dimension, The Hulk has been getting increasingly difficult for SHIELD to handle, with The Hulk going on a series of destructive rampages across several towns and neighborhoods. Nick Fury holds a meeting with other SHIELD Agents and Natasha Romanoff explaining that is he coming up with a solution to hopefully make The Hulk easier to handle. He is working on a new kind of medicine for Bruce which can hopefully control the transformations and have him only turn into The Hulk when they truly need him. Natasha wonders if this plan will truly work, as Hulk is like a force of nature that no being on Earth could possibly match. After the meeting, with the lack of world-ending threats going on lately, Tony Stark (Iron Man) is planning to hold some kind of big party in the Avengers Tower with a bunch of people for "the freaking sake of it" as he says in his own words. After a few hours of planning, the party begins and even Steve Rogers (Captain America) and Peter Parker (Spider-Man) show up.
The party seems to be going well, until an apparent earthquake strikes followed by a massive blue beam destroying some buildings in the Manhattan Financial District. Seeing the carnage from the Avengers Tower, the three heroes immediately suit up and spring into action, and meet up with Natasha and Fury. They try to get a view of what is attacking as people flee the streets in panic, and the culprit behind the destruction is revealed to be a giant radioactive dinosaur. The Avengers retreat, knowing that the creature is too massive to fight without reinforcements. After managing to escape the dinosaur's incoming path of destruction, Thor flies in, looking badly beaten up and has an apparent look of despair on him. Spider-Man asks what's wrong, and Thor reveals that Asgard has been devastated by the same creature that is attacking New York. He says he tried to fight the creature, but it was too powerful. Fury then has a sorrowed look of his own and he reveals that he believes this creature was Godzilla. He explains to The Avengers that he was working on a way to look into other dimensions (such as the Transformers and even DC Comics dimensions) and then he saw Godzilla in his dimension, wreaking havoc and destruction in Tokyo. He explains that Godzilla is a radioactive dinosaur that was mutated by the atomic bomb testings and he attacked and destroyed the city of Tokyo in 1954. Yet he has no idea how Godzilla got there since his dimensional portal plans were scrapped ages ago. Thor suggests they go to Asgard to find out how the monster got there, since he started off in Asgard.
The Avengers reach Asgard and they find in total ruins. While mourning the destruction of his kingdom, Thor spots his evil brother, Loki crawling out of the rubble. Believing Loki may have had something to do this considering his history, he confronts and angrily interrogates his half-brother. Loki, too damaged to fight back, gives in and explains that he was using a magic spell that can help him look into other dimensions with the intent on destroying Thor once and for all. He saw some potential candidates for it, and considered bringing forth some man in blue tights with an "S" on his chest, but he realized he was a good guy and not a villain (in a reference to Superman). However, he then spotted Godzilla. Loki was amazed at the incredible power of this monster, so he used his magic to create a portal to bring Godzilla to Asgard. Loki then tried to gain control of Godzilla's mind through a mind-control spell, but it failed spectacularly as Godzilla's will was too powerful. Godzilla attacked Loki, who manages to escape. But, Godzilla then began to wreak havoc on Asgard and devastated the nation and then made it to New York City using the portal that lets Asgardians get to Earth. Thor berates his half-brother for this decision and states that Loki has a brought to this dimension a destructive force like none that anyone has ever seen.
Meanwhile, Bruce Banner is hiding out in a cottage in a wooded area, as he knows that SHIELD could be looking for him any minute. He decides to turn on the TV and sees the news feed live on TV, and sees Godzilla continuing to tear Manhattan to the ground in a rage, to his absolute horror. The very thought of all the lives that are being lost in the monster's rampage turns him into The Hulk. The Hulk, however, has different intentions from Banner. Hulk is upset that something out there could take away his title of being "the Strongest one there is," and he is refusing to let that happen. He jump his way towards New York, intent on destroying Godzilla. While this is going on, Nick Fury is ordering some S.H.I.E.L.D. Agents and Natasha to help out with evacuation efforts. He is planning an ambush with the Heli-Carrier, and he assigns Iron Man and Thor to lure him into their trap.
Iron Man fires some repulsor blasts at the rampaging Godzilla to get him aggravated, and it works. Godzilla angrily chases Iron Man and Thor in an attempt to destroy them. Iron Man and Thor manage to succeed in luring Godzilla to the ambush, and the Heli-Carrier arrives along with several SHIELD jets. The Heli-Carrier and the SHIELD jets all attack Godzilla with full force, and the attack seems to be going well. However, Godzilla immediately retaliates and blasts the SHIElLD jets to pieces with his atomic breath. He then uses his atomic breath to shoot down the Heli-Carrier, and the Heli-Carrier crashes into the Brookyln area, decimiating entire blocks. A shocked Nick Fury then orders his men that didn't get shot down to help get the survivors out of the wreckage. However, Godzilla then stomps towards the wreckage and is about to fire his breath to kill Nick Fury. He is interrupted, however, by Hulk, who inadvertedly saves the SHIELD director's life. Fury is thankful The Hulk is here for once, but Hulk tells Fury he's not here to save him. He's here for personal revenge. Hulk then begins to battle Godzilla, and even gives the King of the Monsters a tough time. The Avengers then jump back into the battle to aid The Hulk. The battle is intense, but Godzilla then uses his nuclear pulse to blast the Avengers away from him. Iron Man is badly injured and Thor is knocked into a building. Godzilla then proceeds to continue his rampage.
The Avengers, beaten and bruised, try to think of another plan to end Godzilla's reign of terror, but realize they are out of ideas. Nick Fury states he has one more idea. He says he is allowing one of the Avengers the option to volunteer to be exposed to his experimental enlarging ray he's been working on to try and be as big and powerful as Godzilla, so that the enlarged hero can defeat him once and for all. The Hulk immediately volunteers, as he wants revenge on the monster. Nick Fury, cautious considering the Hulk's destructive personality, is initially hesitant, but agrees to do it, as they have no other options left. They head back to the Avengers Tower and they use the enlarging ray on Hulk, causing Hulk to grow rapidly to Godzilla's size. Hulk happily embraces his new size, and then stomps through the city to fight Godzilla. While Godzilla demolishes the Empire State Building with his atomic breath, he spots the 250-foot tall Hulk walking towards him. Hulk tells Godzilla that he has made a grave mistake coming to New York City to cause carnage and destruction, and tells the Big G that no one, not even him, is stronger than Hulk. Hulk lets out a roar of challenge, to which Godzilla immediately accepts.
Godzilla and Hulk engage in a brutal and intense fight. The 2 monsters are equally matched and the Avengers and Spider-Man watch in the Avengers tower as the monstrous combatants go at it. The 2 use everything they have in their arsenal at each other and even use objects in the city as weapons. While this is going on, Nick Fury states he is working on an antidote to the enlarging to get Hulk back to his normal size once he's finished with his battle. He's also having Loki create another portal to bring Godzilla back to his dimension once he's beaten down enough. Meanwhile, Godzilla and Hulk continue their fight and Hulk is starting to lose. Hulk tries to fight back, but Godzilla is too equally matched to keep down. Godzilla quickly gains the upper hand and brutally beats Hulk to a bloody pulp, even punching him so hard Hulk begins to cough up blood. Hulk, too beaten and injured to fight, falls down, and Godzilla unleashes a loud triumphant roar, to the other Avenger's absolute shock and horror. Godzilla then begins to walk towards Avengers Tower, with the intent of eradicating the rest of the team. However, he is interrupted by Hulk, who attempts to use the little strength he has left to fight and unleashes his Critical Thunder Clap, creating a massive blast that decimates 5 city blocks and incapacitates Godzilla.
New York City is in ruins, and Hulk looks down on the injured, defeated Godzilla. Godzilla then finally succumbs to his wounds and seemingly dies. With the battle over, Loki creates another portal and Hulk throws Godzilla in it, sending him back to his dimension. Nick Fury then uses the antidote on Hulk, who returns to his normal size and turns back into Bruce Banner. Nick Fury thanks Bruce Banner for his heroic actions, stating he has not only saved New York City, but possibly the world. Banner then states that if being stuck with The Hulk means keeping the world safe, then he accepts the burden. A few hours later, Hulk jumps across the rooftops of New York City and looks over the ruins of New York City. After a moment, he continues his path, ready to protect the Earth and his title as, "The Strongest One There is."
Meanwhile, back in Godzilla's dimension, Godzilla's body is still laying in the ocean, and Godzilla's eye opens, implying that he never died. He was simply knocked unconscious and that he will live to rampage another day in his dimension.
The film's soundtrack can be played here. As it is a Godzilla movie, the film makes a lot of use of the classic Godzilla soundtrack as well as some Marvel tracks as well.
Cloverfield monster stated in an interview that a sequel to the film is "a possibility, and we are coming up with some sick new ideas for it. The Godzilla universe and the Marvel universe are both so very expansive, there's a lot you could pull from."
It was revealed on January 28th, 2015, that if a sequel gets made, it will feature monsters King Ghidorah, Gigan, and Rodan.
On February 1st, 2015, it was officially announced by Cloverfield that a Godzilla vs Hulk sequel IS in fact going to be put into production, after his other 2 projects, Hulk and the Agents of SMASH (film) and The Amazing Stan Lee.